The skin has barrier functions that allow it to serve as a protective organ to maintain vital activity. In addition, the skin surface may be smooth, dried, wrinkled, etc. Therefore, it can be said that the skin is an organ that plays a cosmetic role. In order to exhibit barrier functions, it is important for the skin to retain moisture mainly with intercellular lipids and natural moisturizing factors in the horny layer. It is known that there is a correlation between the moisture content in the horny layer and skin surface conditions such as smoothness and dryness (see Non-Patent Document 1).
Examples of methods that have been conducted as a means for retaining or improving skin moisture-retaining properties include a method wherein horny layer barrier functions are compensated with the use of a plugging agent such as vaseline ointment or a water-in-oil emulsifying drug formulation; a method wherein the horny layer moisture content is increased with the use of a moisturizing agent such as sorbitol or glycerine; a method wherein skin inflammation is alleviated with the use of an anti-inflammatory drug such as glycyrrhizinic acid; and a method wherein skin cells are activated by vitamins, hormones, and the like (see Non-Patent Document 2).
Psoriasis is chronic inflammatory keratosis with formation of scales. The disease is characterized by excessive growth/abnormal differentiation of epidermal cells, angiogenesis, and invasion of activated T cells (CD3-positive cells) into the epidermis/dermis, etc. For treatment of psoriasis, a variety of anti-inflammatory drugs described above are used and moisturizing agents and the like are used in combination. However, the use of such agents is merely symptomatic treatment. Therefore, there are no radical therapies known to the public.
Pyrroloquinoline quinone (hereinafter referred to as “PQQ”) was discovered as a coenzyme of methanol dehydrogenase contained in a methanol-assimilating bacterium in 1979, (see Non-Patent Documents 3 and 4). PQQ has been detected not only from microorganisms but also from edible plants such as soybeans, broad beans, green pepper, potatoes, parsley, and spinach, and processed food products such as vinegar, tea, cocoa, natto, and tofu (see Non-Patent Document 5). In addition, it has been reported that PQQ is present in humans and rats (see Non-Patent Document 6). Therefore, it is a highly safe substance.
PQQ has been known to have effects such as cell growth promoting effects (see Patent Document 1), effects of removing active oxygen (see Patent Document 2), melanin production inhibitory and skin whitening effects (see Patent Document 3), ultraviolet absorbing effects (see Patent Document 4), and antiallergic effects (see Patent Document 5).
However, it has been unknown that PQQ, an ester thereof, or a salt of either thereof has preventing or ameliorating effects for skin psoriasis.    Non-Patent Document 1: “Archives of Dermatology,” 1985, vol. 121, pp. 642-645    Non-Patent Document 2: “Fragrance Journal” 1999, vol. 10, p. 29    Non-Patent Document 3: “Nature,” 1979, vol. 230, pp. 843-844    Non-Patent Document 4: “FEBS Letters,” 1979, vol. 108, pp. 443-446    Non-Patent Document 5: “Biochemical Journal,” 1995, vol. 307, pp. 331-333    Non-Patent Document 6: “Biochimica et Biophysica Acta,” 1992, vol. 1156, pp. 62-66    Patent Document 1: JP Patent Publication (Kokai) No. 61-58584 A (1986)    Patent Document 2: JP Patent Publication (Kokai) No. 5-078247 A (1993)    Patent Document 3: JP Patent Publication (Kokai) No. 8-020512 A (1996)    Patent Document 4: JP Patent No. 3625493    Patent Document 5: JP Patent Publication (Kokai) No. 63-174931 A (1988)